Electrophotographic dry developing systems employed in copying machines, MFPs or other types of electrophotographic apparatuses are categorized into a system using a one-component developer containing only toner and a system using a two-component developer containing toner and carrier. In either of these developing systems, toner charged to a predetermined level is applied to a photoreceptor. An electrostatic latent image formed on the photoreceptor is rendered visual with the toner and is transferred to a sheet of paper. The image visualized by the toner is fixed on the paper to obtain a desired image.
A brief description about development with the two-component developer will be given. A predetermined amount of toner and a predetermined amount of carrier are accommodated in a developing apparatus. The developing apparatus is provided with a rotatable magnet roller with a plurality of south and north poles alternately arranged thereon in the circumferential direction and an agitation roller for agitating and mixing the toner and carrier in the developing apparatus. The carrier made of magnetic powder is carried by the magnet roller. The magnetic force of the magnet roller forms a straight-chain-like magnetic brush of carrier particles. Agitation produces triboelectric charges that attract a plurality of toner particles to the surfaces of the carrier particles. The magnetic brush abuts against the photoreceptor with rotation of the magnet roller to supply the toner to the surface of the photoreceptor. Development with the two-component developer is carried out as described above.
Fixation of the toner on a sheet of paper results in successive consumption of toner in the developing apparatus, and new toner in the same amount as that of the consumed toner is supplied, whenever needed, from a toner hopper attached to the developing apparatus. On the other hand, the carrier is not consumed for development and is used as it is until the carrier comes to the end of its life. The carrier, which is a component of the two-component developer, is required to have various functions including: a function of triboelectrically charging the toner by agitation in an effective manner; insulation properties; and a toner transferring ability to appropriately transfer the toner to the photoreceptor. To improve the toner's chargeability, for example, the carrier is required to have appropriate electric resistance (hereinafter, sometimes simply referred to as “resistance”) and appropriate insulation properties.
The recently dominating carrier includes carrier core particles, which are the cores or the hearts of the carrier particles, and coating resin that covers the surfaces of the carrier core particles.
The carrier core particles are desired to have good magnetic properties. Briefly speaking, the carrier is carried as described above by magnet rollers with magnetic force in the developing apparatus. Under this usage, if the magnetism, more specifically, the magnetization of the carrier core particles is low, the retention of the carrier to the magnet rollers becomes low, which may cause so-called scattering of the carrier or other problems. Especially, recent tendencies to make the diameter of toner particles smaller in order to meet the demand for high-quality image formation tend to require smaller carrier particles. However, the downsizing of the carrier particles could lead to reduction in the retention of each carrier particle. Effective measures are required to prevent the scattering of the carrier.
Among various disclosed techniques relating to carrier core particles, Japanese Unexamined Patent Application Publication No. 2008-241742 (PTL1) discloses a technique with the aim of preventing the carrier from scattering.